High average power solid-state laser system with phase front control

ABSTRACT

A scalable high power laser system includes a plurality of parallel connected modular power amplifier arms, coupled to a common master oscillator to provide a high average power laser system with a scalable output power level, particularly suitable for laser weapon systems with varying power level output applications. Adaptive optics devices are provided in order to provide pre-compensation of phase front distortions due to the modular amplifier arms as well as encode the wave front of the laser beam with a phase conjugate of atmospheric aberrations.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high average power laser system andmore particularly to a modular high average power laser system whichincludes a phased array of parallel power amplifiers, connected to acommon master oscillator for synthesizing composite beams of varyingpower levels, and adaptive optics which include spatial light modulatorsfor encoding the wave front of the laser beam with a conjugate phase tocompensate for atmospheric aberrations.

2. Description of the Prior Art

High power laser weapon systems are generally known in the art. Anexample of such a high power laser system is disclosed in U.S. Pat. No.5,198,607, assigned to the same assignee as the assignee of the presentinvention and hereby incorporated by reference. Such laser weaponsystems normally include a tracking system for locking the high powerlaser on a target, such as a ballistic missile, cruise missile, bomberor the like. Such laser weapons are used to destroy or “kill” suchtargets. The effectiveness of such a laser weapon system depends on manyfactors including the power of the laser at the target. Many factors areknown to affect the power of the laser at the target. One such factor isknown as thermal blooming, discussed in detail in U.S. Pat. No.5,198,607. In order to compensate for thermal blooming, it is known touse multiple high power lasers for killing a single target, for exampleas disclosed in U.S. patent application Ser. No. 08/729,108, filed onOct. 11, 1996 for a LASER ALONG BODY TRACKER (SABOT) by Peter M.Livingston, assigned to the same assignee as the assignee of the presentinvention.

Other factors are known to affect the power level of the laser at thetarget including atmospheric aberrations which cause distortion of thewave front of the high power laser beam. In order to correct the wavefront of the laser beam due, for example, to atmospheric aberrations,various adaptive optics systems have been developed. Examples of suchsystems are disclosed in U.S. Pat. Nos. 4,005,935; 4,145,671; 4,233,571;4,399,356; 4,500,855; 4,673,257; 4,725,138; 4,734,911; 4,737,621;4,794,344; 4,812,639; 4,854,677; 4,921,335; 4,996,412; 5,164,578;5,349,432; 5,396,364; 5,535,049; and 5,629,765, all hereby incorporatedby reference.

Various laser wave front compensation techniques have been employed. Forexample, U.S. Pat. Nos. 4,005,935; 4,794,344; and 5,535,049 utilizeBrilloin scattering techniques to generate a phase conjugate of thelaser wave front in order to compensate for distortions. Othertechniques include the use of spatial light modulators which divide thelaser beam into a plurality of subapertures, which, in turn, aredirected to an array of detectors for detecting the phase frontdistortion which, in turn is used to compensate the phase fronts as afunction of the distortion. Examples of systems utilizing spatial lightmodulators are disclosed in U.S. Pat. Nos. 4,399,356; 4,854,677;4,725,138; 4,737,621; and 5,164,578, all hereby incorporated byreference.

There are several disadvantages of the systems mentioned above. Onedisadvantage relates to the fact that such laser systems have a fixedarchitecture for a given laser power output level. As such, such lasersystems are generally not scalable. Unfortunately, various laserapplications require different power levels. For example, laser weaponapplications require different output power levels depending on the typeand distance of the intended targets. In such laser weapon applications,separate laser systems are required for each application which increasesthe cost of the laser weapon system as well as the number of spare partsrequired for maintenance.

Another disadvantage of such known laser systems with phase frontcompensation is that such systems are limited to the power level abilityof the various components forming the system. For example, such laserweapon systems are known to use lasers, normally high average powerchemical lasers which have power levels of a few kilowatts. Due to suchhigh power requirements, spatial light modulators have heretofore beenunsuitable for such applications. As such, alternate techniques havebeen developed providing wave front compensation of such high averagepower lasers. For example, U.S. Pat. No. 4,321,550 relates to a highaverage power laser system with phase conjugate correction. In thissystem, the phase front correction is based on Brilloin scattering. U.S.Pat. No. 3,857,356 discloses another system which utilizes a diffractiongrating to provide a reduced power level with test beam. The systemdisclosed in '636 Patent also includes an interferometer with a phaseshifting device disposed in one leg to provide phase front compensationhigh average power laser systems.

Although such systems are suitable for providing phase frontcompensation of high average power laser systems, such systems arerelatively bulky and inefficient. In many applications, there is adesire to use laser weapons that are more efficient and compact,particularly for laser weapon systems.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve various problems inthe prior art.

It is yet another object of the present invention to provide a wavefront compensation system for compensating phase distortions of arelatively high average power level laser systems.

It is yet a further object of the present invention to provide a lasersystem with phase front compensation which is relatively compact andefficient.

It is yet a further object of the present invention to provide a laserpower system with wave front compensation which provides a scalableoutput power level to enable the architecture of laser system to be usedin various laser applications of various power levels.

Briefly, the present invention relates to a scalable high power lasersystem which includes a plurality of power amplifiers coupled to acommon maser oscillator to provide a laser system with a scalable outputpower level, particularly suitable for laser weapon systems with varyingpower level output applications. Adaptive optics are provided in orderto compensate for phase front distortions. The adaptive optics isdisposed on the input of the power amplifiers to providepre-compensation of phase front distortions due to the power amplifiermodules. The adaptive optics also include a spatial light modulator forencoding the wave front with a conjugate phase for compensating for wavefront distortions due to atmospheric aberrations.

DESCRIPTION OF THE DRAWINGS

These and other objects of the present invention will be readilyunderstood with reference to the following specification and attacheddrawings wherein:

FIG. 1 is a generalized block diagram of a laser system in accordancewith the present invention with a scalable power output.

FIG. 2 is a block diagram of a portion of the system illustrated in FIG.1 but with the adaptive optics disposed downstream of the poweramplifiers.

FIG. 3 is similar to FIG. 2 but shown with the adaptive optics disposedupstream of the power amplifiers.

FIG. 4 is a block diagram of a laser system with a scalable power outputlevel which includes phase front compensation for the distortion causedby the power amplifier as well as the atmospheric aberrations inaccordance with the present invention.

FIG. 5 is a block diagram of an exemplary wave front sensor inaccordance with the present invention.

DETAILED DESCRIPTION

The present invention relates to a relatively high average power lasersystem with wave front compensation. The system in accordance with thepresent invention is suitable for use in relatively high average powerapplications making the system suitable for use with laser weaponsystems. An important aspect of the invention is that the system isformed with a scalable architecture which includes a plurality ofparallel power amplifier which enable the output power level to bescaled for different power level applications. As mentioned above,various laser applications, such as laser weapon applications requiredifferent power output levels depending upon the type as well as thedistance of the intended targets. The scalable architecture of the lasersystem in accordance with the present invention is particularly suitablefor laser weapon systems and is also compatible with the power levelcapability of known spatial light modulators for compensation for wavefront distortions of the laser beam resulting from atmosphericaberrations.

The modular laser system with a scalable power output level with wavefront compensation is illustrated in FIGS. 1 and 4 and generallyidentified with the reference numeral 20. As mentioned above, animportant aspect of the invention relates to the fact that the modularlaser system 20 is able to provide for wave front compensation of arelatively high average power laser system, suitable for use in highenergy laser weapon systems. Referring to FIG. 1, the modular lasersystem 20 includes a plurality of modular amplifier arms 22, 24 and 26,connected a common master oscillator 28 forming a scalable high averagepower solid state laser system with wave front compensation inaccordance with the present invention. The modular laser system 20enables the power output level to be scaled while taking advantage ofadaptive optic devices, as will be discussed in more detail below, whichhave relatively limited power level capabilities. More particularly,each modular amplifier arm 22, 24 and 26 includes an adaptive opticsdevice 28, 30, 32, a pre-amplifier 34, 36 and 38 as well as a poweramplifier 40, 42 and 44, all serially coupled. The power output of themodular laser system is scaled by the number of parallel modularamplifier arms 22, 24 and 26 connected to the master oscillator 28.Although three modular amplifier arms 22, 24 and 26 are shown in FIGS. 1and 4, additional modular amplifier arms can be added, limited by thepower capability of the master oscillator 28.

As illustrated in FIGS. 2 and 3, the placement of the adaptive opticsdevices 28, 30 and 32 in the modular amplifier arms 22, 24, and 26allows the system to take advantage of known adaptive optics deviceswhich includes spatial light modulators whose power capability islimited to a few kilowatts. FIGS. 2 and 3 illustrate the differences indisposing the adaptive optics modules 28, 30 and 32 downstream andupstream of the power amplifiers 22, 24 and 26. Both systems illustratedin FIGS. 2 and 3 provide wave front compensation. More particularly,referring to FIG. 2 first, in response to a flat input wave front 46,the output wave front 48 is distorted by the amplifier modules 40, 42and 44. The distorted output wave front 48 from the amplifier modules40, 42 and 44 is corrected by the adaptive optics devices 28, 30 and 32to provide a relatively flat output wave front 49. However, disposingthe adaptive optics devices 28, 30 and 32 downstream of the poweramplifiers 40, 42 and 44 as shown in FIG. 2 results in full powerloading on the adaptive optics 28, 30 and 32. Unfortunately, with atopology as illustrated in FIG. 2, the power capabilities of variousadaptive optics devices including spatial light modulators are exceededfor relatively high average power laser systems. For example, for asystem 20 as illustrated in FIG. 1 with a 12 kilowatt output, eachmodular amplifier arm 22, 24 and 26 would be subjected to 4 kilowattswhich exceeds the power capability of many known spatial lightmodulators. As discussed above, the power capability of known spatiallight modulators is just a few kilowatts. Thus, the topology illustratedin FIG. 2 would be unsuitable for spatial light modulators.

The topology illustrated in FIG. 3 allows the modular laser system 20 totake advantage of known spatial light modulators for wave frontcompensation. In particular, in the embodiment illustrated in FIG. 3,the adaptive optics devices 28, 30 and 32 are disposed upstream of thepower amplifiers 40, 42 and 44. With such a topology, in response to aflat input waveform 46, the adaptive optics devices 28, 30 and 32provide a phase conjugate wave front 50, which, in turn, is applied tothe power amplifiers 40, 42 and 44. The output of the power amplifiers40, 42 and 44 is a flat wave front 52. In the topology illustrated inFIG. 3, using the above example and assuming a 3 kilowatt gain for eachpower amplifier 40, 42 and 44, the adaptive optics devices 28, 30 and 32are subject to a power level of only 1 kilowatt, well within the 2kilowatt range of known spatial light modulators.

Referring back to FIG. 1, the master oscillator 28 provides pulses ofradiation or light into the modular amplifier arms 22, 24 and 26. Themaster oscillator 28 may be a conventional laser, such as a gas laser,dye laser or a solid state laser. The master oscillator 28 is coupled tothe modular amplifier arms 22, 24 and 26 by way of a plurality of beamsplitters 54, 56, 58. The beam splitters 54, 56 and 58 are conventionaland are used to direct a portion of the light beams from the masteroscillator 28 to each of the modular amplifier arms 22, 24 and 26. Foran exemplary 12 kilowatt output laser system as discussed above, themaster oscillator 28 is selected to have about 3 kilowatt output power.

The distributed light pulses from the beam splitters 54, 56 and 58 areapplied to the adaptive optics devices 28, 30 and 32 which, as will bediscussed in more detail below, compensate for optical parameterdistortions of the wave front distortions of the output laser beam atthe target resulting from atmospheric aberrations. The pre-amplifiers34, 36 and 38 amplify the distributed light beam pulse from the masteroscillator 28 which, in turn, is further amplified by the poweramplifiers 40, 42 and 44. The power amplifiers 40, 42 and 44 are used toprovide coherent output beams which, as will be discussed in more detailbelow, can be combined by a beam combiner to provide a scalable highaverage power level output light beam.

The adaptive optics 28, 30 and 32 are discussed in more detail below. Anexemplary pre-amplifier 34, 36 and 38 may be a low-power (1 KW level)amplifier module consisting of a gain medium, such as Nd:YAG slab, andoptical pumping means, such as an array of diode lasers. In the examplediscussed above, the pre-amplifiers 34, 36 and 38 are selected to have again of approximately 20. Each power amplifier 40, 42 and 44 may beselected to consist of three 1 KW module gain sections and provide 3kilowatts of amplification. Suitable power amplifiers 40, 42 and 44 arediode-pumped high-power Nd:YAG slab lasers.

An exemplary high average power solid state laser system 70 isillustrated in FIG. 4. The system 70 illustrated in FIG. 4 includes amaster oscillator 72, for example, a solid state laser, which includesits own adaptive optics device 74 for providing a relatively flat outputwave front. The adaptive optics device 74 for the master oscillator 72may be a slow spatial light modulator for compensating for wave frontphase distortion resulting from the master oscillator 72. An exemplarymaster oscillator 72 consists of a Nd:YAG laser with nearlydiffraction-limited beam quality. An exemplary adaptive optics device 74is a liquid-crystal phase modulator array with electronic means toadjust the phase profile. Such a master oscillator and adaptive opticsare known in the art.

The master oscillator 72 provides a pulsed light beam that isdistributed among a plurality of parallel connected modular amplifierarms 76, 78 and 80 by way of a plurality of beam splitters 82, 84 and86. The distributed pulsed light beams are applied to adaptive opticdevices 88, 90 and 92 which, will be discussed in more detail belowcompensate for optical path distortions resulting from the poweramplifiers as well as distortions of the laser wave front due toatmospheric aberrations to provide a coherent light beam with arelatively flat phase front. The outputs of the adaptive output devices88, 90 and 92 are applied to pre-amplifiers 94, 96 and 98, foramplifying the distributed light pulse on the master oscillator 72. Theoutput of the pre-amplifiers 94, 96 are applied to image relays 100, 102and 104. The image relays 100, 102 and 104 maintain the near field beamprofile from one gain module to the next in order to optimize powerextraction and to prevent potential damage due to beam spillage causedby diffraction. Such image relays are known in the art. An apertureplaced within each relay 100, 102, and 104 also blocks unwanted lightfrom passing through the gain sections that would otherwise createparasitic oscillations. The outputs of the image relays 100, 102, and104 are applied to a plurality of power amplifiers 106, 108 and 110which, as shown, are provided with 3 gain sections 112, 114 and 116. Thepower amplifiers 106, 108 and 110 provide coherent amplified outputbeams 112, 114 and 116 which, may be combined by a beam combiner 118 toprovide a high average power output beam 120. As discussed above, thepower level of the output beam 120 is scalable by the number of modularamplifier arms 76, 78 and 80 included in the system 70.

The wave front of the output beam 120 is detected by a wave front sensor121 which forms a feedback controller in a closed loop with the adaptiveoptics devices 88, 90 and 92 to provide holographic phase conjugation;encode the wave front with a phase conjugate wave which compensates fordistortions of the phase front due to atmospheric aberrations. Eachadaptive optic device 88, 90 and 92 may include a slow spatial lightmodulator 22 and a relatively fast spatial light modulator 124. The slowspatial light modulator 122 provides pre-compensation of relatively slowwave distortions of the light beams due to the power amplifiers 106, 108and 110. The fast spatial light modulators 124 are serially coupled tothe slow spatial light modulators 122 to provide for conjugate waveencoding of the wave front to compensate for distortions due toatmospheric aberrations. Each of the fast spatial light modulators 124may consist of an array of individually addressable pixels. These pixelsunder the control of the wave front sensor 122 are modulated as afunction of wave front of the output beam 120 to create a conjugatephase front.

An exemplary wavefront sensor consists of a Mach-Zehnder interferometerin which a small portion of the master oscillator output provides areference wave to form an interferogram image of the amplifier outputbeams by sampling a small fraction of the output beam, as illustrated onFIG. 5. The interferogram image converts the phase errors into intensityvariations that can be observed and recorded by an electronic photodiodearray or CCD camera and an electronic image capture device (e.g.,computer with frame-grabber and processing software). The resultinginformation on the magnitude of the phase error as represented by imagebrightness at each position of the sampled beam contains the wavefrontdata. The adaptive optics (AO) controller uses this data to generate theconjugate of the wavefront for each pixel of the AO in each amplifierpath.

The AO element consists of a slow and fast parts, driven separately bythe AO controller. The slow AO may consist of liquid-crystal (LC)spatial light modulator (SLM) that has an array of phase shifters withrelatively large dynamic range (several waves) but with slow response(seconds). The fast AO may also be built using a LC-SLM array that isoptimized for smaller range (up to one wave) but much faster response(less than millisecond). The slow and fast components of the wavefrontdata are separated in the processor to drive respective parts of the AOcontroller.

The system 70 illustrated in FIG. 4 may be used to form a high averagepower solid state laser with wave front compensation. In addition tobeing compact and efficient, the high average power level solid statelaser provides a scalable power output useful in applications where thepower level requirements vary. In order to increase the kill level ofsolid state lasers used for laser weapons, the system provides adaptiveoptics for compensating for optical component distortions as well asencoding the phase front with a phase conjugate wave in order tocompensate for atmospheric aberrations.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. Thus, it is to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specificlly described above.

I claim:
 1. A high average power laser system with a scalable outputpower level, the laser system comprising: a master oscillator forgenerating pulsed light beams; one or more modular amplifier arms forproviding an output light beam, each modular amplifier arm opticallycoupled to said master oscillator and including a power amplifier foramplifying said pulsed light beam distributed from said masteroscillator and defining an output light beam; one or more first adaptiveoptics devices for encoding the wave front for said output beam with aphase conjugate to compensate for wave front distortions of said outputbeam due to atmospheric aberrations, said first adaptive optics devicesdisposed in one or more of said modular amplifier arms and including afirst spatial light modulator, said first spatial light modulator beinga relatively fast spatial light modulator for providing holographicphase conjugation; a second adaptive optics device, serially coupled tosaid first adaptive optics device; and a beam combiner for combining theoutput beams for said modular amplifier arms and providing a scalablecomposite output beam whose power level is a function of the number ofmodular amplifier arms connected to the system, said system beingconfigured such that the output level of said scalable composite outputbeams exceeds the power capability of each of said modular amplifierarms.
 2. The laser system as recited in claim 1, wherein said secondadaptive optics device includes a second spatial light modular.
 3. Thelaser system as recited in claim 2, wherein said second spatial lightmodular is a slow spatial light modular for compensating for wavefrontdistortions due to said modular amplifier arms.